1. Field of the Invention
The present invention relates to a spindle unit comprising a working spindle, which is mounted, so as to be rotatable about its longitudinal axis, in its own bearing housing and in which there is provided a receiver for tools for machining workpieces, and to a machine tool comprising at least one such spindle unit.
2. Related Prior Art
A multiplicity of such spindle units and machine tools are known in the prior art.
The known spindle units are used individually or multiply in the known machine tools for the purpose of, for example, chip-removing machining of workpieces. These machine tools have one, two or more working spindles, each of which is mounted in its own bearing housing, also termed spindle housing, the spindle housing or housings being arranged fixedly on a spindle carrier, also termed spindle head.
The spindle head can be moved in three orthogonal axes, designated as the X-axis, Y-axis and Z-axis, relative to a workpiece table, onto which workpieces to be machined can be clamped, in so-termed fixtures. Frequently, yet further relative movements are possible between the working spindle and the workpiece. Thus, in some applications, the spindle head is pivotally mounted, yet further axes also being able to be arranged in the workpiece, which, for this purpose, is held in a fixture that is, for example, rotatable or pivotable.
Whereas such machine tools can be controlled well in respect of control technology, there is nevertheless the requirement for yet ever more rapid workpiece machining, to enable a greater number of workpieces to be machined per unit of time.
This requirement is met, on the one hand, by ever more rapid relative movement capability between the spindle head and the workpiece table. Owing to the large masses to be moved and the therewith associated forces of acceleration, increasing of the travel speed is nevertheless subject to limits if the accuracy and reproducibility of the machining within a workpiece and between workpieces machined in chronological succession by one and the same working spindle are not to be adversely affected.
On the other hand, however, the throughput can also be increased through the use of multispindle machines.
In the case of such multispindle machines, a plurality of workpieces, corresponding to the number of spindles, can be machined synchronously and simultaneously by means of a tool inserted, respectively, in the respective working spindle. The working spindles, and consequently also the tools, are moved synchronously in the three axes of the coordinate system, such that identical machining operations are performed on the workpieces. The throughput of workpieces per machine tool can thereby be increased significantly.
The multispindle machines, however, exhibit problems in respect of accuracy, which ensue from their design. Since the tools cannot be moved separately from one another, it is crucial for the accuracy and reproducibility of the workpiece machining that the spacing of the axes of rotation of the working spindles, and consequently the spacing of the longitudinal axes of the tools, in relation to one another corresponds exactly to the spacing of the tools in relation to one another. This can be achieved in that the fixtures clamping the workpieces are mounted on the workpiece table, and if necessary readjusted, in such a way that they have a center-to-center spacing in relation to one another that corresponds to the spacing of the longitudinal axes of the workpieces and/or to the position of the longitudinal axes perpendicular to this spacing.
Likewise, it is important that the tools have an identical position in the direction of their longitudinal axis, i.e., usually in the direction of the Z-axis in the case of traveling-column machines, such that, for example, they can drive bores of equal depth into the workpieces. It is therefore detrimental to the accuracy and reproducibility, both between workpieces machined simultaneously in parallel and between workpieces machined in chronological succession, if the tools differ in length. This must be taken into account in clamping the tool into the respective tool holder via which the tool is clamped into the receiver in the respective working spindle. In this way, there are always held ready in the corresponding tool magazine sets of tools that match one another in respect of their length. Nevertheless, it is possible for the tools to have differing lengths as a result of differing wear, which then results in machining inaccuracies.
A further source of error in the case of such double-spindle or multispindle machines ensues as a result of thermally caused changes in the position both of the bearing housings in relation to one another and of the fixtures in relation to one another or to the bearing housings. In the course of operation, such machine tools usually become heated, which results in the spacing between the bearing housings, and consequently the working spindles, being increased. Likewise, it is possible for the working spindles to become displaced relative to one another in the Z direction, since the thermal deflections have differing effects upon the different bearing housings. In addition to the displacement caused by the increasing self-heating of the machine tool, such thermal deflections also ensue in the course of the day as a result of, for example, increasing insulation.
Although the temperature changes also affect the spacing of the fixtures in relation to one another, the temperature sensitivity of the workpiece table differs significantly from the temperature sensitivity of the spindle carrier, such that the thermal deflections displace the fixtures in a manner that differs from that in which they displace the bearing housings. A further problem consists in that fixtures are also frequently exchanged during the operation of a machine tool, it being the case that the center-to-center spacing of the newly exchanged-in fixtures can differ from the center-to-center spacing of the fixtures previously in use. This then means that it is necessary for the position of the workpieces, and consequently the position of the fixtures, in relation to one another to be altered and adapted to the position of the working spindles in order for the wanted precision to be ensured in the parallel machining of two or more workpieces. This is very laborious and time-consuming.
Overall, this means that, in the case of machine tools of the type mentioned at the beginning, it is desirable to be able to adjust, i.e., correct, the position of the working spindles in relation to one another in the course of operation, for example in order to compensate the abovementioned thermal deflections.
There are known in the prior art various methods by which the position of the bearing housings can be readjusted in relation to one another in the case of double-spindle or multispindle machines.
DE 103 29 402 A1 describes a machine tool in which the bearing housings are mounted in eccentric bushes, via which the bearing housings can be repositioned, axially parallelwise relative to a central shaft.
DE 198 59 360 A1 describes a machine tool in which the spacing between two bearing housings can be altered by piezoelectric positioning elements, as a result of which the relative position of the bearing housings in relation to one another can be shifted in an axially parallel manner or, also, tilted.
A similar design is disclosed by DE 103 43 320 A1. Here, however, a rib is provided as a positioning element between the bearing housings, the temperature of which rib can be raised or lowered by means of a thermal apparatus for the purpose of thus modifying its length and thereby influencing the relative position of the bearing housings in relation to one another.
DD 34 868 A describes a multispindle drilling head for drilling bores arranged centrosymmetrically on a hole circle. The drilling head has a series of spindle units mounted eccentrically on a spindle carrier, which, in turn, is rotatably mounted in the drilling head. Rotation of the spindle carrier alters the radial spacing between the respective spindle unit and the center of the drilling head, and the circumferential spacing between the individual spindle units.
In the case of all of these machine toots, it has been found to be disadvantageous that the positioning elements do not operate sufficiently rapidly, or affect the accuracy of the machining in an unwanted manner. Further, the repositioning paths are frequently insufficient.